I'd love to see your science that categorically disproves the existence of a God.

The best part about science is that it has never seriously sought to disprove the existence of a god. Some of the greatest scientists of our era actually personally believe or believed that a higher power must exist.

No, you shouldn't force them. You should, however, be willing to pay them more (i.e., overtime) for the time beyond the 40-hour week (or whatever your contracts say is a full week) or any time they continue to work while on vacations. You should also not be holding these "over-dedicated employees" as the new standard.

Of course, being an employer, you have already decided the "right" answer to your question and the asking of it was merely a red herring.